Current practice for the design and construction of overspill dams is such that they are designed for flood conditions (e.g. 1000-year flood) producing very high heads on the sill when spilling (depth of water on the sill of the order of 1 m to 5 m).
For a given flood discharge capacity, an uncontrolled overflow spillway offers greater safety against hydrological uncertainty (one of the most important risks for a dam) than gated discharge works.
Against this advantage, a completely uncontrolled overspill is wasteful of live reservoir capacity, by an amount commensurate with the maximum head of water on the sill, i.e. the difference in elevation between the abovementioned two predetermined levels. The capacity thus lost may represent a significant percentage (as much as or even more than 50%) of total live reservoir capacity, especially for dams of small to moderate size.
The problem which the invention is intended to solve can be summarised in the following two principle goals which may be obtained singly or in combination:
1) To augment the storage capacity of a dam with an uncontrolled overflow spillway, on a near-permanent basis.
2) To retain and/or increase the safety and reliability of operation that is an inherent feature of uncontrolled overflow spillways by permitting the unobstructed passage of major floods while tolerating overspilling by small or moderate floods without outside action or any significant modification to the existing structure.
Various means for augmenting reservoir capacity have been proposed and currently exist. Most consist basically of some system of gates which prevent flow over the sill when closed. Automatically or manually operated, conventional or inflatable gates of all kinds generally exhibit high capital cost and demand routine maintenance and periodic testing. They also demand continuous human supervision or an often costly and sophisticated automatic system controlled by the water level in the reservoir which is never totally free from the risk of breakdown or malfunction. Lastly, for a given discharge capacity, the safety and reliability of operation of gated discharge works are less than for an uncontrolled (ungated) overflow spillway.
There are means of temporarily augmenting reservoir storage capacity such as sandbags or flashboards but they are of limited utility and since they demand human action prior to the arrival of every river flood, they involve a major risk.
Some large embankment dams are provided with `fuse plug` sections topped out at a lower crest elevation than the main dam which operate by erosion of the constituent materials when a very large flood causes a large rise in headwater level. The fuse plug is designed to prevent uncontrolled catastrophic overspilling of a major flood over the main dam by concentrating its effects on a specially prepared section designed to be washed away by erosion to provide extra discharge capacity. Once the fuse plug has been destroyed, major repair works are necessary before the dam can be restored to normal service.
To the applicant's knowledge, there would thus appear to be no means currently in existence of satisfactorily fulfilling the goals stated hereinabove, of simple operation and moderate cost.